X射线断层成像
维基百科，自由的百科全书
X射线断层成像（Computerized Tomography，简称CT），是一種影像診斷學的检查。這一技術曾被稱為電腦軸切面斷層影像（Computed Axial Tomography）。
X射线断层成像是一種利用數位幾何處理後重建的三維放射線醫學影像。該技術主要通過單一軸面的X射线旋轉照射人体，由于不同的組織對X射线的吸收不同，可以用電腦的三維技術重建出斷層面影像。經由窗宽、窗位處理，可以得到相應组織的斷層影像。將斷層影像層層堆疊，即可形成立體影像。
X射线断层成像是一種利用數位幾何處理後重建的三維放射線醫學影像。該技術主要通過單一軸面的X射線旋轉照射人体，由於不同的生物組織對X射線的吸收力(或稱阻射率Radiodensity)不同，可以用電腦的三維技術重建出斷層面影像，經由窗值、窗位處理，可以得到相對的灰階影像，如果將影像用電腦軟體堆疊，即可形成立體影像。
目录 |
[编辑] 診斷應用
自從20世纪70年代被發明後，X射线断层成像在醫學影像上已經變成一個重要的工具，雖然價格昂貴，它至今依然是診斷多種疾病的黃金準則。X射线断层成像技术的优点之一是它可以提供很高的空间分辨率(0.5mm)。它的一个弱点是软组织对比度较差。当诊断对软组织对比度要求较高时，核磁共振影像技术要优于X射线断层成像技术。
[编辑] 頭部斷層檢查
主要用來診斷腦部血管病變以及顱內出血，檢查不一定要用到顯影劑。在病人有急性中風的情形下，它雖然沒辦法排除血管阻塞的可能性，但是可以排除出血的可能性，如此一來，抗凝血劑就可以大膽地應用。在診斷腫瘤的應用上，電腦斷層配合靜脈顯影的檢查並不常用，而且效果也比核磁共振影像(magnetic resonance imaging，簡稱MRI)差。它也可以用來診斷顱內壓是否有增加，例如要做腰椎穿刺前(或是評估ventriculoperitoneal shunt時)。
X射线断层成像在診斷有外傷的顱骨及顏面骨的骨折也有很大的用處。在頭頸口的部位，對於頭骨和顏面骨或是牙齒的畸形，它有術前評估的作用；下顎、副鼻竇、鼻腔，眼框等部位所生囊腫或是腫瘤的評估；慢性鼻竇炎成因的診斷；還有植牙重建的評估。
[编辑] 胸腔斷層檢查
在肺部組織的診斷上，X射线断层成像對於急性或是慢性的變化都有很高的診斷價值，在觀察一些人體內空氣的變化(例如肺炎)或是腫瘤，一般不需顯影劑就有很好的效果了。而一些間質組織的變化(肺實質，肺纖維等等)，可以用薄切面的高解析設定來重建；要評估縱隔腔和肺門部分的淋巴腺腫大，則需要靜脈顯影。
胸腔斷層血管攝影(CTPA)它是一個需要用精確快速的時間來作對比劑注射再加上高速的螺旋式描掃器才能完成的檢查，近來也用在作肺栓塞和動脈剝離的評估。當胸腔x光檢查出現異常或是懷疑異常等，只要是非急性的，電腦斷層都是首推的進一步檢查。
[编辑] 心臟斷層檢查
隨著旋轉時間的减少(時間解析度，目前较先进的X射线断层成像仪的gantry旋转一周的时间通常在0.5秒左右，并在进一步降低)，再加上多斷層切面(multi-slice)的技術(高達64切)，要同時達到高速度和高解析度不再是夢想，目前已經可以清楚地看見冠狀動脈的影像。在掃描的同時，電腦就可以將一連串的數據重建，如此一來，每單一個心臟斷層影像的數據都可以在x光管迴轉完成前重建完成，即使是目前轉速最快的也一樣，但未來是否能取代侵入性檢查「冠狀動脈導入檢查」還是未知數。
心臟的多斷層切面檢查(Multi-slice Computed tomography，簡稱MSCT)有相當性的潛在危險，因為它的劑量相當於500張的胸腔x光，對於乳癌的潛在誘發性目前還有待商榷。診斷為陽性的正確率大約82%，診斷為陰性的正確率大約93%；敏感度大約81%，特異性為94%，【reference】最有價值的是這個檢查的高診斷陰性正確率，因此，如果電腦斷層診斷不出冠狀動脈的疾病的話，病人應該找尋其他可能引起胸腔病灶的原因。
大部分用軟體就可以找尋的病杜都是用以白種人為研究得到的數據來寫的，所以嚴格來說，結果不完全適用在全人種。
雙射源X射线断层成像機，2005年發明，有相當高的時間解析度(Temporal Resolution)，可以減少高速心跳造成的移動假影，閉氣的時間也不用長，對於不方便閉氣的病人或是不適合打降低心率藥的病人是很有幫助的。
[编辑] 腹部和骨盆的斷層檢查
對於腹部的疾病，X射线断层成像的診斷價值極高，常用來定位腫瘤期數也用來做後續的追蹤，對急性腹痛的檢查也很有用。泌尿結石，闌尾炎，胰臟炎，憩室，腹部動脈瘤還有腸阻塞等都是可以由電腦斷層做快速診斷的疾病，它也是第一線用來診斷內部臟器外傷的利器。
口服或是直腸對比劑可視需要使用，稀釋的硫酸鋇(2% w/v)是最常用的，一般用來作大腸透視檢查的鋇劑濃度太高，在斷層影像上反而是假影，如果鋇劑有禁忌上的考量的話(例如懷疑病人是腸受傷)，碘對比劑也是選擇之一，其他種類的就看目標是要對哪一個器官顯影，例如直腸的空氣對比劑(空氣或二氧化碳)用在大腸檢查，或是口服純水用在胃部檢查。
電腦斷層在診斷骨盆的應用上有限制在，特別是女性的骨盆，超音波是一個替代方案。除此之外，它也可以部分應用在腹部掃描(例如看腫瘤)，在評估骨折上也有用處，它也可以用在研究骨質疏鬆症，和骨質密度偵量儀一樣，此兩樣都能偵測骨礦物質的密度(BMD)，也就是骨強度的指標，然而電腦斷層的結果不一定和骨密儀一樣(BMD測量黃金準則)，不但貴，病人接受的劑量又高，所以不常使用。
[编辑] 四肢的檢查
X射线断层成像常用來顯示複雜的骨折，特別是節關附近的骨折，主要是因為它可以將想要看的地方重建出來。
[编辑] 優點和危險性
[编辑] 優於X光影像的部分
首先，X射线断层成像为医生提供器官的完整三维信息，而X光影像只能提供多断面的重叠投影；第二，由於電腦斷層的高解析度，不同組織阻射過所得的放射強度(Radiodensity)即使是小於1%的差異也可以區分出來；第三，由于断层成像技术提供三维图像，依診斷需要不同，可以看到軸切面，冠狀面，矢切面的影像，我們稱它為多平面數位重建(Multi-planar reformated imanging)。除此之外，任意切面的图像均可通过插值技术产生。这给诊和科研带来了极大的便利。
[编辑] 輻射劑量
X射线断层成像被視為中度至高度輻射的診斷技術，雖然技術的進步已經增加了輻射的效率，但是同時為了增加影像品質或為了更複雜的技術，還是有增加劑量的考量，進化過的解析度使電腦斷層可以進行新的研究，可以有更多的優點：例如和傳統血管攝影比，電腦斷層血管攝影可以避免插入靜脈管和靜脈導管；電腦斷層大腸攝影也和大腸鋇劑攝影一樣用來診斷腫瘤，但是劑量更低。其方便性以及可適用的情形不斷增加，使它日漸普及，最近在英国的綜合評估中，電腦斷層佔了所有放射性檢查的7%，但是在2000/2001年間，它佔了總合醫療放射劑量的47%(Hart & Wall, European Journal of Radiology 2004;50:285-291)，過度地使用電腦斷層檢查，不管其他地方怎麼滅，還是會導致總體醫療劑量的上升，在一些特別研究放射劑量的論文還有考量很多因子：掃描的體積，PATIENT BUILD，掃描的數量和型式，還有需要的解析度和影像品質。
参考文献：
The New England Journal of Medicine
Review Article Current Concepts PreviousPrevious Volume 357:2277-2284 November 29, 2007 Number 22 NextNext
Computed Tomography — An Increasing Source of Radiation Exposure David J. Brenner, Ph.D., D.Sc., and Eric J. Hall, D.Phil., D.Sc.
The advent of computed tomography (CT) has revolutionized diagnostic radiology. Since the inception of CT in the 1970s, its use has increased rapidly. It is estimated that more than 62 million CT scans per year are currently obtained in the United States, including at least 4 million for children.1
By its nature, CT involves larger radiation doses than the more common, conventional x-ray imaging procedures (Table 1). We briefly review the nature of CT scanning and its main clinical applications, both in symptomatic patients and, in a more recent development, in the screening of asymptomatic patients. We focus on the increasing number of CT scans being obtained, the associated radiation doses, and the consequent cancer risks in adults and particularly in children. Although the risks for any one person are not large, the increasing exposure to radiation in the population may be a public health issue in the future.
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Table 1. Typical Organ Radiation Doses from Various Radiologic Studies.
CT and Its Use
The basic principles of axial and helical (also known as spiral) CT scanning are illustrated in Figure 1. CT has transformed much of medical imaging by providing three-dimensional views of the organ or body region of interest.
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Figure 1. The Basics of CT.
A motorized table moves the patient through the CT imaging system. At the same time, a source of x-rays rotates within the circular opening, and a set of x-ray detectors rotates in synchrony on the far side of the patient. The x-ray source produces a narrow, fan-shaped beam, with widths ranging from 1 to 20 mm. In axial CT, which is commonly used for head scans, the table is stationary during a rotation, after which it is moved along for the next slice. In helical CT, which is commonly used for body scans, the table moves continuously as the x-ray source and detectors rotate, producing a spiral or helical scan. The illustration shows a single row of detectors, but current machines typically have multiple rows of detectors operating side by side, so that many slices (currently up to 64) can be imaged simultaneously, reducing the overall scanning time. All the data are processed by computer to produce a series of image slices representing a three-dimensional view of the target organ or body region.
The use of CT has increased rapidly, both in the United States and elsewhere, notably in Japan; according to a survey conducted in 1996,2 the number of CT scanners per 1 million population was 26 in the United States and 64 in Japan. It is estimated that more than 62 million CT scans are currently obtained each year in the United States, as compared with about 3 million in 1980 (Figure 2).3 This sharp increase has been driven largely by advances in CT technology that make it extremely user-friendly, for both the patient and the physician.
Figure 2 View larger version (18K): [in this window] [in a new window] Get Slide
Figure 2. Estimated Number of CT Scans Performed Annually in the United States.
The most recent estimate of 62 million CT scans in 2006 is from an IMV CT Market Summary Report.3
Common Types of CT Scans
CT use can be categorized according to the population of patients (adult or pediatric) and the purpose of imaging (diagnosis in symptomatic patients or screening of asymptomatic patients). CT-based diagnosis in adults is the largest of these categories. (About half of diagnostic CT examinations in adults are scans of the body, and about one third are scans of the head, with about 75% obtained in a hospital setting and 25% in a single-specialty practice setting.1) The largest increases in CT use, however, have been in the categories of pediatric diagnosis4,5 and adult screening,6,7,8,9,10,11,12,13 and these trends can be expected to continue for the next few years.
The growth of CT use in children has been driven primarily by the decrease in the time needed to perform a scan — now less than 1 second — largely eliminating the need for anesthesia to prevent the child from moving during image acquisition.4 The major growth area in CT use for children has been presurgical diagnosis of appendicitis, for which CT appears to be both accurate and cost-effective — though arguably no more so than ultrasonography in most cases.14 Estimates of the proportion of CT studies that are currently performed in children range between 6% and 11%.1,15
A large part of the projected increase in CT scanning for adults will probably come from new CT-based screening programs for asymptomatic patients. The four areas attracting the most interest are CT colonography (virtual colonoscopy6,7), CT lung screening for current and former smokers,8,9,10 CT cardiac screening,10 and CT whole-body screening.12,13
Radiation Doses from CT Scans
Quantitative Measures
Various measures are used to describe the radiation dose delivered by CT scanning, the most relevant being absorbed dose, effective dose, and CT dose index (or CTDI).
The absorbed dose is the energy absorbed per unit of mass and is measured in grays (Gy). One gray equals 1 joule of radiation energy absorbed per kilogram. The organ dose (or the distribution of dose in the organ) will largely determine the level of risk to that organ from the radiation. The effective dose, expressed in sieverts (Sv), is used for dose distributions that are not homogeneous (which is always the case with CT); it is designed to be proportional to a generic estimate of the overall harm to the patient caused by the radiation exposure. The effective dose allows for a rough comparison between different CT scenarios but provides only an approximate estimate of the true risk. For risk estimation, the organ dose is the preferred quantity.
Organ doses can be calculated or measured in anthropomorphic phantoms.16 Historically, CT doses have generally been (and still are) measured for a single slice in standard cylindrical acrylic phantoms17; the resulting quantity, the CT dose index, although useful for quality control, is not directly related to the organ dose or risk.18
Typical Organ Doses
Organ doses from CT scanning are considerably larger than those from corresponding conventional radiography (Table 1). For example, a conventional anterior–posterior abdominal x-ray examination results in a dose to the stomach of approximately 0.25 mGy, which is at least 50 times smaller than the corresponding stomach dose from an abdominal CT scan.
Representative calculated organ doses for frequently used machine settings1 are shown in Figure 3A and 3B for a single CT scan of the head and of the abdomen, the two most common types of CT scan. The number of scans in a given study is, of course, an important factor in determining the dose. For example, Mettler et al.15 reported that in virtually all patients undergoing CT of the abdomen or pelvis, more than one scan was obtained on the same day; among all patients undergoing CT, the authors reported that at least three scans were obtained in 30% of patients, more than five scans in 7%, and nine or more scans in 4%.
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Figure 3. Estimated Organ Doses and Lifetime Cancer Risks from Typical Single CT Scans of the Head and the Abdomen.
Panels A and B show estimated typical radiation doses for selected organs from a single typical CT scan of the head or the abdomen. As expected, the brain receives the largest dose during CT of the head and the digestive organs receive the largest dose during CT of the abdomen. These doses depend on a variety of factors, including the number of scans (data shown are for a single scan) and the milliamp-seconds (mAs) setting. The data shown here refer to the median mAs settings reported in the 2000 NEXT survey of CT use.1 For a given mAs setting, pediatric doses are much larger than adult doses, because a child's thinner torso provides less shielding of organs from the radiation exposure. The mAs setting can be reduced for children (but is often not reduced5,19); a reduction in the mAs setting proportionately reduces the dose and the risk. The methods used to obtain these dose estimates have been described elsewhere,20 but software that estimates organ doses for specific ages and CT settings is now generally available.21 Panels C and D show the corresponding estimated lifetime percent risk of death from cancer that is attributable to the radiation from a single CT scan; the risks (both for selected individual organs and overall) have been averaged for male and female patients. The methods used to obtain these risk estimates have been described elsewhere.20 The risks are highly dependent on age because both the doses (Panels A and B) and the risks per unit dose are age-dependent. Even though doses are higher for head scans, the risks are higher for abdominal scans because the digestive organs are more sensitive than the brain to radiation-induced cancer.
The radiation doses to particular organs from any given CT study depend on a number of factors. The most important are the number of scans, the tube current and scanning time in milliamp-seconds (mAs), the size of the patient, the axial scan range, the scan pitch (the degree of overlap between adjacent CT slices), the tube voltage in the kilovolt peaks (kVp), and the specific design of the scanner being used.17 Many of these factors are under the control of the radiologist or radiology technician. Ideally, they should be tailored to the type of study being performed and to the size of the particular patient, a practice that is increasing but is by no means universal.19 It is always the case that the relative noise in CT images will increase as the radiation dose decreases, which means that there will always be a tradeoff between the need for low-noise images and the desirability of using low doses of radiation.22
Biologic Effects of Low Doses of Ionizing Radiation
Mechanism of Biologic Damage
Ionizing radiation, such as x-rays, is uniquely energetic enough to overcome the binding energy of the electrons orbiting atoms and molecules; thus, these radiations can knock electrons out of their orbits, thereby creating ions. In biologic material exposed to x-rays, the most common scenario is the creation of hydroxyl radicals from x-ray interactions with water molecules; these radicals in turn interact with nearby DNA to cause strand breaks or base damage. X-rays can also ionize DNA directly. Most radiation-induced damage is rapidly repaired by various systems within the cell, but DNA double-strand breaks are less easily repaired, and occasional misrepair can lead to induction of point mutations, chromosomal translocations, and gene fusions, all of which are linked to the induction of cancer.23
Risks Associated with Low Doses of Radiation
Depending on the machine settings, the organ being studied typically receives a radiation dose in the range of 15 millisieverts (mSv) (in an adult) to 30 mSv (in a neonate) for a single CT scan, with an average of two to three CT scans per study. At these doses, as reviewed elsewhere,24 the most likely (though small) risk is for radiation-induced carcinogenesis.
Most of the quantitative information that we have regarding the risks of radiation-induced cancer comes from studies of survivors of the atomic bombs dropped on Japan in 1945.25 Data from cohorts of these survivors are generally used as the basis for predicting radiation-related risks in a population because the cohorts are large and have been intensively studied over a period of many decades, they were not selected for disease, all age groups are covered, and a substantial subcohort of about 25,000 survivors26 received radiation doses similar to those of concern here — that is, less than 50 mSv. Of course, the survivors of the atomic bombs were exposed to a fairly uniform dose of radiation throughout the body, whereas CT involves highly nonuniform exposure, but there is little evidence that the risks for a specific organ are substantially influenced by exposure of other organs to radiation.
There was a significant increase in the overall risk of cancer in the subgroup of atomic-bomb survivors who received low doses of radiation, ranging from 5 to 150 mSv27,28,29; the mean dose in this subgroup was about 40 mSv, which approximates the relevant organ dose from a typical CT study involving two or three scans in an adult.
Although most of the quantitative estimates of the radiation-induced cancer risk are derived from analyses of atomic-bomb survivors, there are other supporting studies, including a recent large-scale study of 400,000 radiation workers in the nuclear industry30,31 who were exposed to an average dose of approximately 20 mSv (a typical organ dose from a single CT scan for an adult). A significant association was reported between the radiation dose and mortality from cancer in this cohort (with a significant increase in the risk of cancer among workers who received doses between 5 and 150 mSv); the risks were quantitatively consistent with those reported for atomic-bomb survivors.
The situation is even clearer for children, who are at greater risk than adults from a given dose of radiation (Figure 4), both because they are inherently more radiosensitive and because they have more remaining years of life during which a radiation-induced cancer could develop.
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Figure 4. Estimated Dependence of Lifetime Radiation-Induced Risk of Cancer on Age at Exposure for Two of the Most Common Radiogenic Cancers.
Cancer risks decrease with increasing age both because children have more years of life during which a potential cancer can be expressed (latency periods for solid tumors are typically decades) and because growing children are inherently more radiosensitive, since they have a larger proportion of dividing cells. These risk estimates, applicable to a Western population, are from a 2005 report by the National Academy of Sciences25 and are ultimately derived from studies of the survivors of the atomic bombings. The data have been averaged according to sex.
In summary, there is direct evidence from epidemiologic studies that the organ doses corresponding to a common CT study (two or three scans, resulting in a dose in the range of 30 to 90 mSv) result in an increased risk of cancer. The evidence is reasonably convincing for adults and very convincing for children.
Cancer Risks Associated with CT Scans
No large-scale epidemiologic studies of the cancer risks associated with CT scans have been reported; one such study is just beginning.32 Although the results of such studies will not be available for some years, it is possible to estimate the cancer risks associated with the radiation exposure from any given CT scan20 by estimating the organ doses involved and applying organ-specific cancer incidence or mortality data that were derived from studies of atomic-bomb survivors. As discussed above, the organ doses for a typical CT study involving two or three scans are in the range in which there is direct evidence of a statistically significant increase in the risk of cancer, and the corresponding CT-related risks can thus be directly assessed from epidemiologic data, without the need to extrapolate measured risks to lower doses.33
The estimated lifetime risk of death from cancer that is attributable to a single "generic" CT scan of the head or abdomen (Figure 3C and 3D) is calculated by summing the estimated organ-specific cancer risks. These risk estimates are based on the organ doses shown in Figure 3A and 3B, which were derived for average CT machine settings.1
Although the individual risk estimates shown in Figure 3 are small, the concern about the risks from CT is related to the rapid increase in its use — small individual risks applied to an increasingly large population may create a public health issue some years in the future. On the basis of such risk estimates and data on CT use from 1991 through 1996, it has been estimated that about 0.4% of all cancers in the United States may be attributable to the radiation from CT studies.2,34 By adjusting this estimate for current CT use (Figure 2), this estimate might now be in the range of 1.5 to 2.0%.
Conclusions
The widespread use of CT represents probably the single most important advance in diagnostic radiology. However, as compared with plain-film radiography, CT involves much higher doses of radiation, resulting in a marked increase in radiation exposure in the population.
The increase in CT use and in the CT-derived radiation dose in the population is occurring just as our understanding of the carcinogenic potential of low doses of x-ray radiation has improved substantially, particularly for children. This improved confidence in our understanding of the lifetime cancer risks from low doses of ionizing radiation has come about largely because of the length of follow-up of the atomic-bomb survivors — now more than 50 years — and because of the consistency of the risk estimates with those from other large-scale epidemiologic studies. These considerations suggest that the estimated risks associated with CT are not hypothetical — that is, they are not based on models or major extrapolations in dose. Rather, they are based directly on measured excess radiation-related cancer rates among adults and children who in the past were exposed to the same range of organ doses as those delivered during CT studies.
In light of these considerations, and despite the fact that most diagnostic CT scans are associated with very favorable ratios of benefit to risk, there is a strong case to be made that too many CT studies are being performed in the United States.35,36 There is a considerable literature questioning the use of CT, or the use of multiple CT scans, in a variety of contexts, including management of blunt trauma,37,38,39,40 seizures,41 and chronic headaches,42 and particularly questioning its use as a primary diagnostic tool for acute appendicitis in children.14 But beyond these clinical issues, a problem arises when CT scans are requested in the practice of defensive medicine, or when a CT scan, justified in itself, is repeated as the patient passes through the medical system, often simply because of a lack of communication. Tellingly, a straw poll35 of pediatric radiologists suggested that perhaps one third of CT studies could be replaced by alternative approaches or not performed at all.
Part of the issue is that physicians often view CT studies in the same light as other radiologic procedures, even though radiation doses are typically much higher with CT than with other radiologic procedures. In a recent survey of radiologists and emergency-room physicians,43 about 75% of the entire group significantly underestimated the radiation dose from a CT scan, and 53% of radiologists and 91% of emergency-room physicians did not believe that CT scans increased the lifetime risk of cancer. In the light of these findings, the pamphlet "Radiation Risks and Pediatric Computed Tomography (CT): A Guide for Health Care Providers,"44 which was recently circulated among the medical community by the National Cancer Institute and the Society for Pediatric Radiology, is most welcome.
There are three ways to reduce the overall radiation dose from CT in the population. The first is to reduce the CT-related dose in individual patients. The automatic exposure-control option45 on the latest generation of scanners is helping to address this concern. The second is to replace CT use, when practical, with other options, such as ultrasonography and magnetic resonance imaging (MRI). We have already mentioned the issue of CT versus ultrasonography for the diagnosis of appendicitis.14 Although the cost of MRI is decreasing, making it more competitive with CT, there are not many common imaging scenarios in which MRI can simply replace CT, although this substitution has been suggested for the imaging of liver disease.46
The third and most effective way to reduce the population dose from CT is simply to decrease the number of CT studies that are prescribed. From an individual standpoint, when a CT scan is justified by medical need, the associated risk is small relative to the diagnostic information obtained. However, if it is true that about one third of all CT scans are not justified by medical need, and it appears to be likely,35 perhaps 20 million adults and, crucially, more than 1 million children per year in the United States are being irradiated unnecessarily.
Supported by grants from the National Cancer Institute (R01CA088974, to Dr. Brenner), the National Institute of Allergy and Infectious Diseases (U19AI67773, to Dr. Brenner), and the Department of Energy Low Dose Radiation Research Program (DE-FG-03ER63441 and DE-FG-03ER63629, to Dr. Hall).
No potential conflict of interest relevant to this article was reported.
Source Information
From the Center for Radiological Research, Columbia University Medical Center, New York.
Address reprint requests to Dr. Brenner at the Center for Radiological Research, Columbia University Medical Center, 630 W. 168th St., New York, NY 10032, or at djb3@columbia.edu.
References
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Mitelman database of chromosome aberrations in cancer. Cancer Genome Anatomy Project, 2007. (Accessed November 5, 2007, at http://phstwlp1.partners.org:3204/Chromosomes/Mitelman.) 24. Brenner DJ, Doll R, Goodhead DT, et al. Cancer risks attributable to low doses of ionizing radiation: assessing what we really know. Proc Natl Acad Sci U S A 2003;100:13761-13766. [Free Full Text] 25. Health risks from exposure to low levels of ionizing radiation — BEIR VII. Washington, DC: National Academies Press, 2005. 26. Preston DL, Pierce DA, Shimizu Y, et al. Effect of recent changes in atomic bomb survivor dosimetry on cancer mortality risk estimates. Radiat Res 2004;162:377-389. [CrossRef][ISI][Medline] 27. Preston DL, Shimizu Y, Pierce DA, Suyama A, Mabuchi K. Studies of mortality of atomic bomb survivors. Report 13: Solid cancer and noncancer disease mortality: 1950-1997. Radiat Res 2003;160:381-407. [CrossRef][ISI][Medline] 28. Pierce DA, Preston DL. 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Berrington de Gonzalez A, Darby S. Risk of cancer from diagnostic X-rays: estimates for the UK and 14 other countries. Lancet 2004;363:345-351. [CrossRef][ISI][Medline] 35. Slovis TL, Berdon WE. Panel discussion. Pediatr Radiol 2002;32:242-244. [CrossRef][ISI] 36. Donnelly LF. Reducing radiation dose associated with pediatric CT by decreasing unnecessary examinations. AJR Am J Roentgenol 2005;184:655-657. [Free Full Text] 37. Ruess L, Sivit CJ, Eichelberger MR, Gotschall CS, Taylor GA. Blunt abdominal trauma in children: impact of CT on operative and nonoperative management. AJR Am J Roentgenol 1997;169:1011-1014. [Free Full Text] 38. Navarro O, Babyn PS, Pearl RH. The value of routine follow-up imaging in pediatric blunt liver trauma. Pediatr Radiol 2000;30:546-550. [CrossRef][ISI][Medline] 39. Renton J, Kincaid S, Ehrlich PF. Should helical CT scanning of the thoracic cavity replace the conventional chest x-ray as a primary assessment tool in pediatric trauma? An efficacy and cost analysis. J Pediatr Surg 2003;38:793-797. [CrossRef][ISI][Medline] 40. Kaups KL, Davis JW, Parks SN. Routinely repeated computed tomography after blunt head trauma: does it benefit patients? J Trauma 2004;56:475-480. [ISI][Medline] 41. Maytal J, Krauss JM, Novak G, Nagelberg J, Patel M. The role of brain computed tomography in evaluating children with new onset of seizures in the emergency department. Epilepsia 2000;41:950-954. [CrossRef][ISI][Medline] 42. Lewis DW, Dorbad D. The utility of neuroimaging in the evaluation of children with migraine or chronic daily headache who have normal neurological examinations. Headache 2000;40:629-632. [CrossRef][ISI][Medline] 43. Lee CI, Haims AH, Monico EP, Brink JA, Forman HP. Diagnostic CT scans: assessment of patient, physician, and radiologist awareness of radiation dose and possible risks. Radiology 2004;231:393-398. [Free Full Text] 44. Radiation risks and pediatric computed tomography (CT): a guide for health care providers. Rockville, MD: National Cancer Institute. (Accessed November 5, 2007, at http://phstwlp1.partners.org:3191/cancertopics/causes/radiation-risks-pediatric-CT.) 45. McCollough CH, Bruesewitz MR, Kofler JM Jr. CT dose reduction and dose management tools: overview of available options. Radiographics 2006;26:503-512. [Free Full Text] 46. Semelka RC, Armao DM, Elias J Jr, Huda W. Imaging strategies to reduce the risk of radiation in CT studies, including selective substitution with MRI. J Magn Reson Imaging 2007;25:900-909. [CrossRef][ISI][Medline]
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[编辑] 對比劑的負面反應
由於X射线断层成像相當依賴靜脈注射的對比劑來顯影，所以有潛在的危險，危險雖低，卻無法完全避免，這可能會使某些病人的腎臟受傷，如果是有腎功能衰竭或糖尿病等病史的病人，(另外還有REDUCED INTRAVASCULAR VOLUME)危險性可能更高。
[编辑] 影像處理
X光斷層面的數據是由X光射源繞物體一圈得來，感應器是放置於射源的對角位置，隨著物體慢慢地被推入內側端，數據也不斷地處理，經由一系列的數字運算，也就是所謂的斷層面重建來得到影像。
[编辑] 窗宽
所謂的窗宽(windowing)就是指用韓森費爾德(發明者)單位(Hounsfield Unit，簡稱HU)所得的數據來計算出影像的過程，不同的的放射強度(Raiodensity)對應到256種不同程度的灰階值，這些不同的灰階值可以依CT值的不同範圍來重新定義衰減值，假設CT範圍的中心值不變，定義的範圍一變窄後，我們稱為窄窗位(Narrow Window)，比較細部的小變化就可以分辨出來了，在影像處理的觀念上，我們稱為對比壓縮。例如我們為了要在腹內找出肝腫瘤的細微變化，就要用肝窗位，假設70HU是肝脏的平均值(稱為肝窗位)，我們就可以在更窄的窗寬內重新定義範圍，窗位(Window)定為170HU，85HU為上，85HU為下，如此一來範圍就是-15HU到+155HU，低於-15HU的指就顯示全黑，高於+115HU的指就顯示為全白，同理，骨的窗位就要用寬窗位(Wide Window)，主要是考慮到含有脂肪的髓腔內的髓質還有外層緻密骨，當然HU的中心值就大約要用百位的數字了。
[编辑] 三維重建
三維重建指用数学的方法从断层成像仪测量到的信号（X射线通过人体后的衰减）恢复（重建）出器官的三维影像。最简单的，也是最早的，重建方法是反投影法（backprojection）。反投影法虽然直观上很容易理解，但它在数学上是不正确的。目前常用的重建方法主要有两种：滤波反投影法（filtered backprojection）和卷积反投影法（convolution backprojection）。
[编辑] 图像显示
由於目前的X射线断层成像都是等方性(x，y，z軸的解析度都一樣)或是接近等方性的解析度，顯示的方式不一定只限於橫切面，所以，藉著軟體的幫忙，只要把所有的小體素堆疊起來，就可以用不同的視點來看影像。
多層面重建MPR(Multi-Planar Reconstruction)
這是重建最簡單的方式，是把所有的橫切面數據堆疊起來，軟體可以用不同的平面來切割物體（大部分是垂直面），或是特別的一些影像例如最大強度投射成像MIP(Maximum-Intensity Projection)或是最低強度投射成像mIP(Mininum-Intensity Projection)。
多層面重建最常用來檢查脊椎，因為軸切面的影像只限於有時才能顯出椎體，也無法完全秀出椎間盤，經由重組影像，我們可以更容易觀察出脊椎的位置以及其和其他器官的關係。
現代的軟體可以重建斜位的影像，所以經由自由的選擇平面，我們可以看到想看的解剖構造，比如支氣管不是垂直的，我們可以藉由這個技術達到我們要的目的。
在血管的影像上，彎曲的平面也有辦法重建。這使得彎曲的血管可以被「拉直」，如此整條血管可以用一張影像或是少數影像就可以完全顯現，一旦血管被拉直後，量化的長度和寬度就測量出來，對於手術和侵入性治療的幫忙不小。
MIP重建加強了高射束的區域，用在血管攝影很有用，mIP重建趨向於加強空氣的顯示，用來評估肺部結構很有用。
[编辑] 三維呈像技術(3D rendering techniques)
[编辑] 表面呈像(surface rendering)
放射強度(Radiodensity)的閥值是可以調整的(例如對應於骨頭的值)，當閥值一定，便可使用「邊緣偵察(edge detection)」影像處理法，如此一來，一個三維的物體就可以呈像了，不同的物體可以用不同的閥值呈像，使用不同的顏色來代表不同的解剖構造，例如骨，肌肉和軟骨，然而，在這個基礎下，再深一層的構造可能就無法顯像了。
[编辑] 體素呈像(volume rendering)
表面呈像只限於在一定的閥值下，表現物體的表面像，也止於呈現接近我們想像的表面，而在體素呈像中，利用透明度和顏色可以在單一影像中的特色，就可以呈現更多的東西，例如：骨盆就可以用半透明的方式顯現，那麼即使是斜位角，小部分其他的解剖呈像並不會擋住其他重要的部分。
[编辑] 影像分割(Segmentation)
有一些部位雖然結構不同，但是有相似的阻射性，只是單純地改變體素呈像的參數可能不是這麼簡單就可以區分它們，解決的方式我們稱為影像分割(segmentaion)，就是用手動或是自動的方式去除我們不想要的部分。
[编辑] 例子
下面是一些腦部X射线断层成像的影像，骨頭的部分比週圍的地方白(白代表高阻射率)，血管處(箭頭)比較亮是因為使用了碘對比劑的關係。
[编辑] 歷史
第一個商業化的X射线断层成像系統是由Godfrey Newbold Hounsfiled發明的，地點在英國Hayes的THORN EMI Central Research Laboratories，Hounsfield在1967年開始了他的想法，於1972正式發表，聲稱電腦斷層是披頭四樂團最大的遺產，龐大的利益使得EMI投資了研究計劃。另一頭，TUFTS大學的Allen Mcleod Cormack 獨立研發了類似的處理程序，地點是University of Cape Town/Groote Schuur Hospital，他們於1979年一起獲得諾貝爾獎。
1971所產的原型是行經180度角取160個平行讀數，每個是一度，每次掃描大約費時五分鐘，整個影像要產生要花2.5小時並用大型電腦來進行運算。
第一個生產的X射线断层成像掃描器稱為EMI描掃器，只能用來做頭部的掃描，但是要花四分鐘取數據，七分鐘重組完成一個影像，另外它還要用一個裝滿水的perspex容器，型為頭套狀，可以包覆整個頭，主要是為了減少頭部的對比阻射強度相差太大(頭骨和頭骨外的差異)，當時的解析度不高，只有80*80的畫質，第一個EMI掃描器是安裝在英國的wimbledon的atkinson morley's hospital，第一次進行病人頭部檢查的時間是1972年。
在美國，此機器的售價是390000，第一個是安裝在lahey clinic，再來是massachusetts general hospital，還有1973在george washington大學。
第一個任何部位都能檢查且不用水頭套的電腦斷層儀是在goergetown university由robert s.ladley. dds設計。
[编辑] 電腦斷層機器的演進
[编辑] 第一代
用如筆頭般細的射束打向一個或兩偵檢器，影像是用translate rotate的方法，將射源和偵檢器放置於對側的位置，兩者相對位置不變，再加以旋轉。在EMI描掃器時代，一對影像須要旋轉180度，耗時四分鐘，使用三個偵檢器(其中一個是射源位置的參考)，每個偵檢器都是由碘化鈉閃礫器和光電倍增管組成，部分的病人很不能適應這些早期的機器，因為機器的振動和聲音都太大了。
[编辑] 第二代
這項設計增加了偵檢器的數目，並且改變了射束的形狀，把原本的筆頭型改為扇型，旋轉方式仍為translate rotate，但是掃描時間有明顯的減少，旋轉量也由每次一度增為每次三十度。
[编辑] 第三代
第三代X射线断层成像在獲得影像的時間上有長足的進步，扇形的射束配上一列和射源相對的偵檢器，省略了費時的translation stage，最初讓掃描時間減少至大約一張十秒鐘，這個進行讓ct的實用性大大增加，時間短到可以做肺部和腹部的掃描，之前的幾代只限於用在頭部和四肢，到了第三、四代，病人也明顯覺得噪音和振動都少了不少，舒適多了。
[编辑] 第四代
它的設計方法幾乎和第三代是同時發明的，表現度也差不多，不用一列的偵檢器，取而代之的是360度整圈的偵檢器，用扇型射束旋轉打在固定而非旋轉的偵檢器上。
bulky是一項昂貴且脆弱的光電倍增管，所以漸漸地被較好的偵檢器取代，氙游離腔偵檢器列曾經用在第三代機器中，也增加了較多的解析度和敏感度，但最終這兩項技術都被固態偵檢器取代：一個矩形、固態的發光二極體，並鍍上瑩光的稀土元素磷，它更小，更敏感，更穩定，也更適合第三、四代機器的設計。
早期的四代機器有600個光電倍增管，每個直徑1/2吋，可以套在偵檢環內，以三個發光二極體為單位可以替代一個光電倍增管，這項改變同時增加了取像速度和影像品質，但是掃描的速度仍然不能改善，因為x光管的控制還是用纜線啟動，限制了旋轉的速度。
一開始，第四代機器有一個重大的進步，就是每轉一圈，偵檢器就會自動校正一次；而三代的幾何方式固定，對於沒有校正的情形很敏感，也就是有環形假影產生的可能，另外，四代由於偵檢器不會移動和振動，校正的執行也較容易。
所有現代的醫療用電腦斷層都是以第三代的設計為藍本，現代的固態偵檢器相當地穩定，可以不須要每掃一個影像都校正一次，第四代由於偵檢器經濟效益的問題，使得它比第三代貴多了，甚至對假影的敏感度也高，因為沒有固定和射源相對的偵檢器，要去除散射幾乎是不可能的事。
[编辑] 第五代
一般指的是所謂的攝影CT（cine-CT）；Cine-CT與第四代CT相似，但X光源被置於偵辦器的外環；而且為了加快掃瞄的速度，採用多管X光源，依序以不同位置之X光對剖面曝光，以取代旋轉功能。系統掃瞄速度因而大大提升，足以掃瞄心跳等動態的剖面圖。而真正所謂第五代CT，乃是以大角度陽極X光管，環繞掃瞄剖面與偵測器；利用電子方式控制撞擊陽極的電子束，使其發出不同角度的X光束，以達到如同多管X光源的效果。由於電子掃瞄速度極快，每一剖面的掃瞄時間可降至33ms-100ms左右。。適用於心導管，做心臟、血管攝影，主要缺點劑量高，價格昂貴。
[编辑] 功能再進化
和取象時間有關，要克服的另一問題是x光管，要提供一個長時間，高強度的曝露，須要將非常穩定的輸出加到x光管和發電器中，高速的迴轉陽極要跟上處像處理的速度，需要固定150kV的SMPS才能趨動他們，目前的動力強度可以到100kW
環刷迴轉(slip-ring)技術取代了原本纜線的設計，始得x光管和偵檢器能連續動作，再加上連續地推移病人進入掃描器的設計，就是所謂的螺旋式電腦斷層。
多层螺旋X射线断层成像(Multi-Detector-Row Computed Tomography，簡稱MDCT)的系統更加快了掃描的速度，它可以同時獲取數個影像，目前的機器列數可以到64列，要在幾秒內就有完整的胸腔影像也是有可能的，以前的檢查假設要分十次閉氣，一次十秒，現在可能一次十秒的閉氣可以完成了。MDCT也是使用等方解析度，可用任意的角度重建你想要的影像，和核磁共振影像有一樣的能力，在很短的時間就可以掃描很大的體積是MDCT最大的特色，然而更重要的事是空間解析度也要高，最新一代MDCT內在Z軸方向的球管內有浮動的焦班，可以讓解析度更好，另一個不同的方向的研究是用在心臟的斷層檢查，稱為電子光束斷層描掃(Electron-Beam Computed Tomography，簡稱EBCT)，時間解析度高達50微秒，它可以暫停心臟和肺部的動態來形成高品質的影像，只有Imatron公司有製造，後來GE公司跟進，鮮有人做，主要是因為它的成本太高，而且設計的用途只有一項而已，同期的MDCT其時間解析度就很接近EBCT了，但是成本低得多，也因為如此，MDCT就成了市場的趨向。
進化過的電腦技術和組像技術可以執行更快更準確的重組，早期的機器可能要幾分鐘才一張影像，現在則是三十秒就可以做出1000張影像，精心設計的軟體已經可以滅少假影了。雙射源電腦斷層(Dual source)使用了兩個x光管和兩排偵檢器，使得每張影像只要0.1秒就可以完成，如此就可以得到高品質的心臟影像而不需要用降低心率的藥，例如beta blockers。
雙射源的複列偵檢器電腦斷層可以在十秒的閉氣時間內就完成整個心臟的檢查。
Volumetric電腦斷層是複列偵檢斷層機的一項延申，仍在研究階段，目前的MDCT每轉一次取樣4cm寬的體積，volumetric電腦斷層的目標是以256的複列偵檢斷層儀的原型為基礎，增加寬度到10-20cm，未來的應用包括了心臟成像(在兩次連續的心跳間就可以取得欲重建完整三維影像所需要的數據)。
[编辑] 微斷層攝影(Microtomography)
近幾年來，斷層攝影也到了微米的等級，名為微斷層攝影，但是這些機器目前只適合小物體或是動物，還不能用在人體。
